The shooting, which left four students and an adult injured at the Sal Castro Middle School, is being considered "unintentional," said Los Angeles Police Department spokesman Tony Im.

Detectives are continuing to investigate the shooting. Lt. Chris Ramirez with the LAPD said negligent can mean many things, but most likely a "mishandling."

"This continues to be an active investigation," according to a LAPD statement. "However, at this time, the information suggest that this was an isolated incident, involving the negligent discharge of a firearm, where innocent children and a staff member were unfortunately injured."

The girl has been taken to the Los Angeles County Central Juvenile Hall. Police didn't say what led to the shooting and little information was released about the girl who was detained. She was taken into custody without incident, officials had said.

Meanwhile, the middle school, which is in the city's Westlake district, will have classes Friday.

Injured, but expected to recover

The shooting was reported to police Thursday at 8:55 a.m. It happened in an elective class with students from different grades, said the LA School Police Chief Steven Zipperman.

The 12-year-old girl and witnesses were being interviewed, he said Thursday.

The victims include a 15-year-old boy who was shot in the head, but is expected to recover, said officials at Los Angeles County + USC Medical Center. He was shot in the temple and was "extremely lucky" in that the bullet missed vital structures, said Dr. Aaron Strumwasser, a trauma surgeon. The student remains in intensive care.

A 15-year-old girl who was shot in the wrist was also hospitalized. Two other students, ages 11 and 12, suffered graze wounds, were treated and released, Strumwasser said.

A 30-year-old woman suffered a minor injury in the chaos after the shooting, officials said.

On Thursday, after the campus was declared safe, the school day continued and students had the chance to talk to counselors about the shooting, said Vivian Ekchian, interim superintendent of the Los Angeles Unified School District.

Police confiscated a gun at the scene. Ramirez said the weapon was a semiautomatic.

Questions of how and why the gun ended up at the school remain.

The grandmother of the 12-year-old girl spoke briefly with CNN affiliate KABC. She was not identified, but told the station that her granddaughter was caring and loving, but didn't have many friends and had been bullied by another girl.